It is more than a decade since the first report on carbon nanotube as a new material. The exceptional mechanical and electrical properties of carbon nanotube have attracted intensive attention of physicists, chemists and material scientists worldwide, however, its commercial application has not been realized yet. The reasons lie in two interrelated aspects: the difficulty in mass production of carbon nanotubes and hence the high production cost. For instance, the international market price of carbon nanotubes of 90% purity is as high as $60/g, which is 5 times that of gold. It is reported that the highest production rate of carbon nanotubes till now is only 200 g/h (MOTOO YUMURA et al., CNT10, October 2001, p. 31). There are also reports forecasting that industrial application of carbon nanotubes will remain unpractical until its price falls below $2/pound, i.e. 0.4 cent/g, and it needs a production rate of 10,000,000 pound per year or about 12.5 tons per day to bring the price down to this level. Thus, in order to take carbon nanotubes from laboratory to market, mass production of high-quality carbon nanotubes is one of the principal challenges to take.
Novel process and reactor technology are the keys to the mass production of carbon nanotubes. Known methods for the preparation of carbon nanotubes mainly comprise graphite arc-discharge method, catalytic arc-evaporation method and catalytic decomposition method, of which the catalytic decomposition method is the most prevalent, especially the catalytic decomposition of lower hydrocarbons. The production of carbon nanotubes by chemical vapor deposition is a process involving both a typical chemical engineering process and the special preparation process of nanometer materials. Thus a desired production method should meet the requirements of heat transfer and mass transfer in the chemical engineering process while taking the special properties of nano-materials into consideration. Carbon nanotubes are one-dimensional nano-materials that grow during the reaction, and which demand a catalyst with its active ingredients dispersed on the nano-scale and which need sufficient space for growth. For a high rate of reaction, an appropriate concentration of catalysts is also necessary.
The gas-solid fluidization technique is an efficient measure to intensify the contact between gases and solids and has been widely used in many fields, and it is particularly suitable for the preparation, processing and utilization of powders. The gas-solid fluidization technique offers many advantages, such as high throughput, large capacity of transporting/supplying heat, and easy transfer of powder products and catalysts. However, traditional gas-solid fluidized beds are only used for the fluidization of non-C-type powders with diameters larger than 30 μm (Geldart D. Powder Technology, 1973,7: 285). The growth of one dimensional materials and their adherence to each other in the preparation of carbon nanotubes by chemical vapor deposition tend to make fluidization difficult, and thus cause coagulation, uneven distribution of temperature and concentrations, and the deposition of carbon among particles. Therefore, there has been no report on the application of fluidized-bed reactor in continuous mass production of carbon nano-materials.
It is now known that the inter-particle forces among fine powders do not monotonically increase with the decrease of particle sizes. The intense Van der Waals force among nanometer particles can be effectively weakened in some nanomaterial systems by the formation of structurally loose agglomerates by the self-agglomeration of primary particles, which makes the said nano-materials fluidized and capable of flowing in the form of agglomerates. Chaouki et al. (Powder Technology, 1985, 43: 117) have reported the agglomerate fluidization of a Cu/Al2O3 aerogel. Wang et al. (Journal of Tsinghua University, Science and Technology Engineering, vol.41, No.4/5, April 2001, p32-35) investigated the particulate fluidization behaviors of SiO2 nano-agglomerates. The agglomerate fluidization of carbon fibers was also reported by Brooks (Fluidization V, New York: Engineering Foundation, 1986, pp217).